Under abuse conditions, high energy density electrochemical cells can leak or rupture which can cause damage to the device employing the cell or to people using the device. Examples of abuse conditions for a cell are abuse charging of the cell, forces discharging and external shorting. Such conditions cause the internal temperature of the cell to rise and a corresponding increase in pressure. Although such cells typically employ a venting mechanism wherein the electrolyte is expelled, the electrolyte can itself cause damage. Additionally, in cells which employ lithium as an anode material, if internal cell temperatures reach above 180.degree. C., the lithium can melt and result in a fire. Therefore, safety devices other than venting means which will disconnect the electrical circuit under abuse conditions are desirable. If the circuit can be disconnected, the rise in the cell's internal temperature as a result of the abuse conditions can be terminated.
One method of interrupting the flow of electrical current in the cell when the internal temperature of the cell rises is to employ a PolySwitch disc, tradename of Raychem Corp., in the cell. Such a disc is a resettable device which, when a threshold temperature is reached, will inhibit the flow of electrical current in the cell. When the temperature is lowered, the disc will allow the normal available current in the electrical circuit to be restored. Unfortunately, these devices are not suitable for all applications. For example, when the PolySwitch disc is designed to function at low temperatures, the disc may have a low breakdown voltage. Once the breakdown voltage is reached, the disc no longer inhibits the current flow. Therefore, if cells containing these discs are used in a multicell device, and abuse conditions are experienced, the breakdown voltage would be reached and the PolySwitch disc would not inhibit the current flow.
In Japanese Patent Publication No. 59-191273, a memory alloy terminal lead is employed in a lithium/thionyl chloride cell. When the internal temperature of the cell rises, the alloy lead wire folds down to break the electrical circuit. Forming a disconnect member of this single lead wire is undesirable, because the member must be trained to fold down, which requires great expense and time. Also, the single strip can require a relatively large amount of cell space. This is undesirable because all of the available cell space does not then contain active cell components.
In U.S. Pat. No. 4,035,552, a bimetallic strip is disposed in the space between the cell cover and cell casing and functions as a thermal and pressure responsive switch. Because the switch is employed in the space between the cover and case, this switch also unfortunately occupies a relatively large amount of cell space.
In view of the disadvantages of known methods, it would be desirable to have an electrochemical cell which has safety-switch means which will work reliably under substantially all abuse conditions.
Also it will be desirable to have such a means which would not take up an undesirable amount of cell space.